1. Field of the Invention
The present invention pertains to turbo machines which supply or recover the energy of a multiphase fluid, and to vary the pressure thereof.
2. Description of the Prior Art
The pumping, and also the expansion of a multiphase fluid constituted by at least one liquid phase and a gaseous phase, poses a problem which is difficult to resolve.
For example, in the case of the compression of a multiphase fluid, experience shows that the performance of the classic centrifugal pumps slumps as soon as the level of gas exceeds a few percent.
The best adapted classic rotodynamic pumps do not allow gas levels of 20 to 25% to be exceeded without the rise in pressure falling considerably. The other types of pumps, such as the reciprocating displacement pumps or screw pumps, jet-effect pumps, are also limited in their uses and their energy efficiency.
Various pumping devices have been developed in the prior art in an attempt to improve the performance of the pumps working with multiphase flow.
For example, French Patents 2,333,139, 2,471,051 and 2,665,224 disclose hydraulic cells for axial or quasi-axial pumps which exhibit blading and inter-blade channel geometries for the pumping of multiphase fluids. These cells ensure both limitation of the accelerations and good homogenisation of the fluid which are essential elements for obtaining good performance with diphase flow. The pumps are composed of one or more cells of this type, mounted in successive stages on the rotating shaft.
French Patent 2,743,113 describes a device comprising blading disposed in tandem to ensure the passage of the liquid phase from the front side to the back side and to improve the mixing of the liquid and gaseous phases in the flow channels.
German Patent 2,287,288 is an example of a stator for an axial multiphase pump designed to alternate the direction of rotation of the multiphase flow between the outlet of the rotor and the inlet of the stator. Such an arrangement makes it possible to improve the mixing of the liquid and gaseous phases.
U.S. Pat. No. 5,628,616 describes impellers for a semi-radial or "mixed flow" type pump comprising openings which allow the recirculation of the gaseous and liquid phases in order to ensure their mixing.
The communication entitled "Innovative Solutions for Multiphase Pumping", presented in June 1995 at the "Multiphase 95" 7.sup.th international conference discloses a counter-rotating compressor for wet gas comprising impellers turning in the opposite direction around the same axis so as to improve the mixing of the gaseous and liquid phases.
The rise in pressure in multiphase flow obtained through such devices reaches by way of example 30 to 80% of the rise in pressure which would be obtained with a monophase fluid with a density equal to the mean density of the mixture.
Obtaining good performance with a multiphase pump comes in large part from its capacity for intimately mixing the liquid and gaseous phases. However, in the current state of the art, the multiphase flow in the cells remains practically parallel with the surfaces of the blading, the housing and the hub of the hydraulic cells.